In pregnancies complicated by various factors such as diabetes and other high risk factors or complications, the possibilities of the newborn infant being delivered prior to the development of lungs mature enough to autonomously support respiration may be increased significantly. As a result, reliable tests for the determination of fetal pulmonary maturity are desirable and would eliminate some of the risk in the delivery of such a pulmonarily immature infant. Pulmonary immaturity is commonly referred to as "respiratory distress syndrome" (RDS) and continues to rank as a major public health problem.
Analysis of the amniotic fluid is commonly used as a clinical test of fetal maturity. The most widely accepted method has been to determine the ratio of lecithin and sphingomyelin (L/S ratio). Other more recent studies have advocated the analysis for phosphatidyl glycerol, which is recognized as a potent phospholipid and surfactant in the amniotic fluid and is generally analyzed by a thin layer chromotography method.
Other phospholipids have also been measured with chromatography as a means for predicting the risk of RDS.
It is now widely agreed that RDS in infants is brought on by the collapse of the pulmonary alveoli. The resistance of pulmonary alveoli to collapse during expiration has been shown to be due to the presence of a surfactant, i.e., a substance which lowers surface tension in the alveoli. It has subsequently been confirmed that a surfactant is secreted into the alveoli, and further that the production and secretion of surfactant into the alveoli is dependent upon reaching a certain gestational age, i.e., maturity.
Pulmonary surfactant development also correlates with the stability of bubbles in amniotic fluid mixed with ethanol, a diluent in a solution which is used in a test known as a Foam Stability Index (FSI) test.
The above named method of analysis of fetal pulmonary maturity measures the stability or life of bubbles in an amniotic fluid solution. The FSI test includes a combination of amniotic fluid and ethanol shaken vigorously for thirty seconds, and observation of the existence or nonexistence of an uninterrupted ring of bubbles at the air-fluid interface around the meniscus of the test tube, following a settling period of fifteen or more seconds after shaking. This test, while generally providing reliable positive results, is found to yield false negative results and therefore cannot be completely accurate in screening for immature fetal pulmonary function. When compared with the L/S test, which takes several hours, much expertise and sophisticated instruments to perform, and further, is not usually available on a twenty-four emergency basis, the FSI test is far superior. The FSI test is much simpler, is not dependent on the availability of sophisticated equipment and can be performed and analyzed in less than thirty minutes. However, the reliability of the foam stability test depends greatly on the addition of precise amounts of both the amniotic fluid and the alcohol. As a result the test must be performed under controlled conditions, and, even when so conducted, the test results do not reliably predict immature lung function.
In view of the above current test procedures for fetal pulmonary maturity, the ideal test should be rapid and inexpensive, yet have a high accuracy or predictive level for both mature and immature pulmonary function.